Remove leading underscore in __test_sink, __test_source, etc.

Summary:
The naming convention for these test functions will break if we start handling private names correctly in classes, see docs on private name mangling: https://docs.python.org/3/reference/expressions.html?highlight=mangling

At runtime, it's impossible to access a function named with a leading double underscore from inside a class:

```
>>> def __toplevel(x: int) -> int:
...     return x
...
>>> class Foo:
...     def test(self):
...             print(__toplevel(1))
...
>>> Foo().test()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 3, in test
NameError: name '_Foo__toplevel' is not defined
```

This diff is a no-op except adjusting the naming of these test helpers to lead with a single underscore instead.

Includes: `__test_sink`, `__test_source`
still need to do: `__global_sink`, `__user_controlled`, `__tito`

Reviewed By: gbleaney

Differential Revision: D30150334

fbshipit-source-id: 271814d20ef9df6984531bffe8158151cfd91574

Author: shannonzhu

source/analysis/test/globalResolutionTest.ml

@@ -2502,7 +2502,7 @@ let test_define context =
   in
   let source =
     {|
-    from builtins import __test_sink
+    from builtins import _test_sink
     from typing import Callable
 
     def simple_function(x: int) -> str:
@@ -2512,14 +2512,14 @@ let test_define context =
     def with_logging(callable: Callable[[str], None]) -> Callable[[str], None]:
 
       def inner(y: str) -> None:
-        __test_sink(y)
+        _test_sink(y)
         callable(y)
 
       return inner
 
     def two_inner_functions(callable: Callable[[str], None]) -> Callable[[str], None]:
       def inner1(y: str) -> None:
-        __test_sink(y)
+        _test_sink(y)
 
       def inner2(y: str) -> None:
         inner1(y)
@@ -2530,7 +2530,7 @@ let test_define context =
     def decorator_factory(x: int) -> Callable[[Callable[[str], None]], Callable[[str], None]]:
       def wrapper(f: Callable[[str], None]) -> Callable[[str], None]:
         def inner(y: str) -> None:
-          __test_sink(y)
+          _test_sink(y)
           f(y)
 
         return inner
@@ -2554,12 +2554,12 @@ let test_define context =
     ~source
     ~expected_define_source:
       {|
-    from builtins import __test_sink
+    from builtins import _test_sink
     from typing import Callable
     def with_logging(callable: Callable[[str], None]) -> Callable[[str], None]:
 
       def inner(y: str) -> None:
-        __test_sink(y)
+        _test_sink(y)
         callable(y)
 
       return inner
@@ -2569,12 +2569,12 @@ let test_define context =
     ~source
     ~expected_define_source:
       {|
-    from builtins import __test_sink
+    from builtins import _test_sink
     from typing import Callable
 
     def two_inner_functions(callable: Callable[[str], None]) -> Callable[[str], None]:
       def inner1(y: str) -> None:
-        __test_sink(y)
+        _test_sink(y)
 
       def inner2(y: str) -> None:
         inner1(y)
@@ -2587,13 +2587,13 @@ let test_define context =
     ~source
     ~expected_define_source:
       {|
-    from builtins import __test_sink
+    from builtins import _test_sink
     from typing import Callable
 
     def decorator_factory(x: int) -> Callable[[Callable[[str], None]], Callable[[str], None]]:
       def wrapper(f: Callable[[str], None]) -> Callable[[str], None]:
         def inner(y: str) -> None:
-          __test_sink(y)
+          _test_sink(y)
           f(y)
 
         return inner